DESCRIPTION (adapted from the Abstract): The objectives of this project are to analyze the representation of acoustic features of complex sounds in the central auditory system, and the relation of this neural processing to vocal communication in the natural environment. The auditory system of anuran amphibians will be used as a model system for understanding how neural processing in the brain underlies and guides natural acoustic behavior. This application outlines a series of experiments using physiological, anatomical, and behavioral techniques designed to test specific hypotheses of the role of temporal acoustic features in organizing and driving responses of auditory brainstem nuclei, and in mediating acoustic communication behaviorally. Physiological experiments will focus on neural coding of temporal features of complex sounds in two auditory nuclei, the torus semicircularis in the midbrain and the dorsolateral nucleus in the medulla. The goal of these experiments is to examine the operation of two competing neural codes for temporal information processing, one based on changes in spike rate of individual neurons and one based on the ability of individual neurons to phase lock or synchronize to the period of complex sounds. Competing hypotheses of how the periodicity or pitch of complex sounds is extracted in the central nervous system using different time domain algorithms will also be tested. The organization and representation of periodicity information in the midbrain and medulla will be analyzed by neuronal tract-tracing and cell filling techniques. These experiments will examine interconnections of functionally-described areas of these nuclei, and how these interconnections might modulate neural response properties. Of particular interest is the examination of different kinds of spatial representation of pitch information, as discrete periodotopic maps or as anatomical gradients of changes in neural codes representing pitch. These experiments will also examine how differences in cell morphology might reflect differences in acoustic information processing. Behavioral experiments using the evoked calling technique will be conducted to identify the roles of temporal properties of complex sounds in guiding vocal communication in the natural environment. These experiments directly impact upon theories of the neural bases for periodicity pitch perception in humans. Pitch is a critical dimension of auditory perception in humans, playing an important role in the identification, localization and interpretation of many kinds of sounds, including speech and music. The physiological bases for pitch perception has long been one of the dominant themes in research on hearing, but many questions about the neural extraction of pitch remain unanswered. Because pitch perception is so fundamental to hearing in humans, understanding its neural bases will be of value in the development of improved auditory prosthetic devices.